The present invention relates to a method of forming a protective film for an optical recording medium such as a compact disk (CD), a video disk or a magneto optical recording disk.
In manufacturing an optical recording medium such as compact disks (CD), small holes serving as information pits are formed at least in one surface of a substrate, for example, a transparent substrate formed of polymethyl methacrylate resin (PMMA) or polycarbonate resin (PC), in molding the substrate, an evaporated metallic film serving as a reflective film or a recording layer is formed over the surface having the information pits, and a protective film having a thickness in the range of several microns to several tens microns and formed of a transparent resin, such as an ultraviolet ray curing resin or lacquer, is formed over the evaporated metallic film.
The substrate having the information pits is formed by a stamping process using a stamper having protrusions corresponding to the information pits by an injection molding process using the same stamper disposed in a mold or by a so-called 2P process (photoplymerization process) in which an ultraviolet ray curing resin is filled between a flat resin plate having good optical characteristics and a stamper, and then the ultraviolet ray curing resin is irradiated by ultraviolet rays to cure the resin.
Generally, the protective film is formed by a spincoating method. However, the spin-coating method entails various problems. For example, as shown in FIG. 3, in forming the substrate by an injection molding process, a stamper 1 having a surface carrying a pattern of pits complementing the pattern of information pits is placed on a movable mold 2, a fixed mold 3 is held over the movable mold 2 so as to form a cavity 4 between the movable mold 2 and the fixed mold 3, and then a resin is injected through a sprue 5 into the cavity 4. In FIG. 3, indicated at 6 and 7 are a die and a punch, respectively, for forming a central hole in the substrate. Since the stamper 1 is held along the inner and outer peripheries thereof by stamper holders 8 and 9, an annular groove 12 as shown in FIG. 4 corresponding to the stamper holder 8 is formed in the molded substrate 11 having information pits 10 respectively corresponding to the protrusions of the stamper 1. Then, as shown in FIG. 5 or 6, a reflective film or a recording layer 13, such as an aluminum film formed by vacuum evaporation, having a thickness in the range of 600 .ANG. to 1500 .ANG. is formed over the surface having the information pits 10 of the substrate 11, and then a protective film 14 is formed over the recording layer 13 to complete an optical recording medium, for example, a CD 16. Indicated at 17 is the central hole.
In forming the protective film 14 of the CD having a construction as shown in FIG. 5, the outer peripheral region and inner peripheral region including the annular groove 12 of the substrate 11 are covered selectively with a mask 15 as shown in FIG. 7 by way of example, the reflective film or the recording layer 13 is formed selectively in the region in which the information pits 10 are formed, for example, by an aluminum evaporation process, the mask 15 is removed, and then the substrate 11 is rotated and an ultraviolet ray curing resin, for example, is dropped on the substrate 11 at a position indicated by an arrow a in FIG. 5, namely, a position radially outside the annular groove 12 and radially inside the region in which the reflective film or the recording layer 13 is formed, to form the protective film 14. Since the protective film 14 is formed over the entire surface of the reflective film or the recording layer 13 including the circumference of the reflective film or the recording layer 13, the reflective film or the recording layer 13 can surely be protected. However, as mentioned above with reference to FIG. 7, troublesome work required for forming and removing the mask 15 is an obstacle to mass production.
On the other hand, in forming the protective film 14 of the CD having a construction as shown in FIG. 6, the reflective film or the recording layer 13 is formed over the entire region having the information pits 10 of the substrate 11 including the annular groove 12, for example, by an aluminum evaporation process without using any mask 15, and then the protective film 14 is formed similarly over the entire region having the information pits 10 by a spin-coating process. In this case, the troublesome work for forming and removing the mask 15 is unnecessary. However, radial irregularities appear in the protective film 14 in a region outside the annular groove 12 due to the adverse effect of the annular groove 12 entailing the degradation of the commercial value and the edges of reflective film 13 are not covered by the protective film 14, which results in the erosion of the reflective film 13 from the edges.